Fault protection system for a respiratory conduit heater element

ABSTRACT

A fault protection circuit for a respiratory conduit heater element in a respirator humidification system is disclosed. The circuit includes a spark detector as well as overcurrent detector. Several variations are included for the spark detector including a two winding transformer, a centre tapped two winding transformer), and a high pass filtered inductor. A semiconductor switching configuration is also disclosed. Once the protection circuit detects a change in current over a certain level, or the average level raises above a threshold, then the current in the heater element is interrupted for a preset period.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to respiratory humidifiers and heatedbreathing conduits used to couple a patient to the humidifier. A cultprotection system for the conduit heater wire is disclosed.

[0003] 2. Description of the Prior Art

[0004] In order to supply gases to a patient or a person needing suchgases, it may sometimes be necessary to first humidify those gases, forexample using a respiratory humidifier/ventilator system. In such a casewhere the gases have been humidified, and therefore laden with water, itis likely that during transport through a conduit to the patient,condensation of that water vapour will occur. In order to overcome thisdisadvantage it is known to associate a heater wire with respiratoryhumidifier breathing conduits to avoid condensation. Examples of such aheated breathing conduit are disclosed in U.S. Pat. No. 5,537,996(McPhee) and U.S. Pat. No. 5,392,770 (Clawson et al.).

[0005] However there are safety concerns with using a heated wiresystem, especially when the gas in the breathing conduit contains a highconcentration of oxygen, which may be a common condition in hospitals.It is possible for ignition of the heater wire and conduit materials tooccur if certain fault conditions are present. Thus physicians may behesitant to use a humidifier with an associated heater wire, due to theperceived risks to the patient. However if the a humidifier is not usedvarious respiratory problems can occur due to the lack of controlledhumidity.

BRIEF SUMMARY OF THE INVENTION

[0006] It is an object of the present invention to provide a faultprotection system for a respiratory conduit heater element which goessome way towards overcoming the abovementioned disadvantages.

[0007] Accordingly, in a first aspect, the present invention consists ina fault protection system for a respiratory conduit heater elementcomprising:

[0008] detecting means which include means to detect a rapid change ofcurrent in said heater element,

[0009] current interruptor means in series with said heater element, and

[0010] timer means adapted to control the action of said interruptormeans and thereby in use determining the duration of currentinterruption, said timer means being triggered by said detecting means.

[0011] In a second aspect, the present invention consists in asemiconductor switching circuit for rapidly controlling the AC supplycurrent through a load from a power supply comprising:

[0012] two sane channel MOSFETs connected in series with their sourceand gate electrodes respectively tied together, said circuit adapted toreceive a switching voltage between the commoned gate and sourceelectrodes, their drain electrodes adapted to be connected to a load anda power supply respectively.

[0013] In a third aspect, the present invention consists in asemiconductor switching circuit for rapidly controlling the AC supplycurrent through a load from a power supply comprising:

[0014] two same channel MOSFETs connected in series with their drain andgate electrodes respectively tied together, said circuit adapted toreceive a switching voltage between the commoned gate and drainelectrodes, their source electrodes adapted to be connected to a loadand a power supply respectively.

[0015] In a fourth aspect, the present invention consists in a faultprotection system for a respiratory conduit heater element comprising:

[0016] a detector means which includes a peak current detector fordetecting current in the heater element

[0017] a current interruptor in series with said heater element, and

[0018] a timing circuit adapted to control the action of said currentinterrupter and thereby in use determining the duration of currentinterruption, said timing circuit being triggered by said detector, whena predetermined threshold current is exceeded.

[0019] In a fifth aspect, the present invention consists in arespiratory humidification system wherein a conduit connects a patientto a humidifier, said conduit being heated by a respiratory conduitheater element controlled by said humidifier, the improvement comprisingthat said humidifier includes a fault protection system for said heaterelement comprising:

[0020] detecting means which include means to detect a rapid change ofcurrent in said heater element,

[0021] current interruptor means in series with said heater element, and

[0022] timer means adapted to control the action of said interruptormeans and thereby in use determining the duration of currentinterruption, said timer means being triggered by said detecting means.

[0023] In a sixth aspect, the present invention consists in arespiratory humidification system wherein a conduit connects a patientto a humidifier, said conduit being heated by a respiratory conduitheater element controlled by said humidifier, the improvement comprisingthat said humidifier includes a fault protection system for said heaterelement comprising:

[0024] detecting means which includes a peak current detector fordetecting current in the heater element

[0025] current interrupter means in series with said heater element, and

[0026] timer means adapted to control the action of said interruptormeans and thereby in use determining the duration of currentinterruption, said timer means being triggered by said detecting means,when a predetermined threshold current is exceeded.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Preferred forms of the invention will now be described withreference to the accompanying drawings:

[0028]FIG. 1 is a schematic of a prior art heated breathing conduit tobe used with a respiratory humidifier,

[0029]FIG. 2 is a block diagram of a spark detector using a two windingtransformer,

[0030]FIG. 3 is a block diagram of a spark detector using a two windingtransformer with a centre tapped primary,

[0031]FIG. 4 is a block diagram of a spark detector using a single coiland high pass filter,

[0032]FIG. 5 is a block diagram of a current level detector,

[0033]FIG. 6 is a circuit diagram of a prior art single MOSFET switchingcircuit, and

[0034]FIG. 7 is a circuit diagram of a pair of back to back MOSFETs usedfor switching the heater wire.

DETAILED DESCRIPTION

[0035] An example of a prior art heated breathing conduit 102 for usewith a respiratory humidifier/ventilator, is shown generally in FIG. 1.The heated breathing conduit ordinarily comprises an inspiratory conduit101 connected at its proximal end 102 to the gases outlet of arespiratory humidifier (not shown) and at its distal end 103 to a “Y”shaped connector having three inlet/outlet ports. One port 104 of the“Y” shaped connector directs the inspiratory gases to the patient andalso receives exhaled air from the patient. The expired air is channeledby the “Y” shaped connector to an expiratory conduit 105 via the thirdport 106 of the “Y” shaped connector so that the expiratory gases may bereturned to the humidifier/ventilator (not shown) from the end 107 ofthe expiratory conduit 105.

[0036] The conduits 101,105 are heated by a heater wire 108 locatedwithin the inspiratory conduit 101 and a second heater wire 108A islocated within the expiratory conduit 105. In this example the twoheater wires are configured in parallel such that the second heater wire108A shares connection 109 with the first heater wire 108 and isconnected at point 123A to the second earth return conductor 121A whichextends from point 123A to connection 110, although other arrangementsare equally possible.

[0037] Power is supplied via standard domestic or industrial supply 114.The heater wire 108 is supplied with power from the secondary side of astep down transformer 118 which is connected to the external voltagesupply across the phase 115 and neutral 116 conductors. A controller 119controls a switch 120 which, when closed, energises the heater wire.

[0038] The controller can determine if there is no heater wireconnected, and provides an audible alarm if this is detected.

[0039] As with all electrical installations there exists faultconditions which potentially can ignite a fire. Trials have indicatedthat two fault conditions in particular appear to be especiallyimportant in starting a fire in a heated respiratory conduit. These are:

[0040] 1. A break in the heater wire, leading to repeated sparks whichcause ignition

[0041] 2. Excessive current in the breathing circuit, leading to meltingor ignition of the breathing circuit materials. This can be caused byincorrect breathing circuit design or assembly, or by a short circuit.

[0042] The present invention may be retrofitted to existing respiratoryhumidifier/ventilator systems or included as part of the humidifiercontroller. It detects sparking and over current in the heating wire 103as detailed in the following embodiments.

[0043] According to the present invention spark detection isaccomplished by detecting the rapid change in current that occursfollowing disconnection of the heater wile load. An inductor is usedbecause a rapid change in current induces a voltage spike across theinductor, which can easily be detected. Since sparks (or disconnectionsof the heater wire) can happen at any part of the mains cycle (includingthose times when the mains voltage is near zero) the spark detectioncircuitry cannot hope to pick up every single spark. In practice thoughdisconnections which occur near the mains zero voltage do not havesignificant energy for ignition. It is practical to detect 75% of allheater wire disconnections, and this provides the required degree ofsafety.

[0044] In a first embodiment of the invention a two winding transformeris connected in series with the heater wire. As can be seen in FIG. 2during normal operation the current flows from the AC supply 1 throughthe primary 2 of the spark detection transformer then through the heaterwire 3. If a break 4 occurs in the heater wire 3 (causing sparking), arapid change in current will occur in the heater wire 3. Any rapidchange in current through the primary winding 2 of the transformercreates a voltage spike, due to the action of its inductance. Thevoltage spike is passed onto the secondary winding 5 of the transformerand is multiplied by the turns ratio.

[0045] The voltage spike on the secondary winding may be either positiveor negative, but the four diodes form a bridge rectifier 6, so that thevoltage spike always charges the capacitor 7 with a positive voltage. Ifthe magnitude of the voltage spike is large enough, then the capacitorwill charge above the threshold voltage 8 of the comparator 9, whichcauses the output of the comparator 9 to go high, enabling the timer 10.This comparator drives a timer 10 which turns off the power to theheater wires for a period of tire (e.g. several seconds) using aswitching circuit 11. The resistor 12 across the capacitor 7 allows thevoltage to decay away to zero if no sparks are detected.

[0046] In a second embodiment a two winding transformer with a centretapped primary winding is connected in series with the heater wire. Inorder to reject mains voltage spikes as a source of false triggering,the primary winding can be centre tapped 18 as shown in FIG. 3, and thecurrents from the inspiratory 13 and expiratory 14 heater wires are eachpassed through a different half of the primary winding. In this way, thecurrent of any mains borne interference passes through both halves ofthe primary winding, and the resultant magnetic fields (flowing throughthe core of the transformer) cancel out. A spark 4 will only occur inone heater wire limb at a time (shown here as the inspiratory 13), andtherefore is not cancelled out. The remainder of the circuit operates inthe same way as the first embodiment, with the capacitor 7 voltagecompared against a threshold 8, and the comparator 9 driving a timer 10.

[0047] In a third embodiment shown in FIG. 4, as an alternative to usinga transformer a coil 15 is connected in series with the heater wire 3.The coil 15 is used instead of the transformer primary utilised in thefirst and second embodiments. A high pass filter (resistors 17 andcapacitors 16) is used to reject mains eg: 50-60 Hz frequencies. Theremainder of the circuit operates in the same way as the firstembodiment. Similarly to the first embodiment this embodiment does notinclude specific rejection of mains borne interference eg: spikes.However variations in this embodiment can be envisaged which doincorporate rejection of mains borne interference.

[0048] The technique used for detecting excess current is common to allthree embodiments of the invention described above. The threshold forcurrent detection is set to be the maximum current that will be drawn bythe lowest foreseeable resistance heater wire, at the highest ratedmains voltage+10%. The current detector is designed to respond to thepeak current instead of the average current for two reasons: (a) peakcurrent is faster responding than average, (b) the peak current isindependent of the duty cycle that the controlling humidifier issupplying.

[0049] Referring to FIG. 5 the heater wire current to be measured ispassed through a low value resistor 24. The voltage which appears acrossthe resistor is proportional to the current flowing in the heater wire23. This voltage is passed through an amplifier 25, then passes to apeak detection circuit, where a capacitor 27 is charged up by a diode 26to the maximum peak of the AC voltage. If the peak voltage is higherthan the threshold voltage 8 then the comparator 29, operates a timer30, which removes power from the heater wire for a period of time. Atthe end of this time period the current is restored, but if the currentis still too high then the peak is detected very quickly and powerremoved again Using this circuit, a completely short-circuited heaterwire can be tolerated without blowing the heater base fuse 60. Theresistor 28 slowly discharges the capacitor.

[0050] The control strategy for all faults involves disconnecting thepower from the heater wire for a period and then reapplying it. This isto avoid shutting the system down in response to a non critical event.

[0051] Common to all embodiments of spark detection is the controlstrategy used for removing and reapplying power to the heater. Thetiming circuit (10) must operate for long enough that any heat that isgenerated by a spark has time to dissipate before the heater wire poweris reapplied. In practice a time period of 1 second has been found to besufficient. As a further aspect, the timing circuit could also be madeto count up the number of sparks detected, and then disconnect theheater wire permanently. This is to discriminate against one of theheater wires being disconnected while the system is in use.

[0052] If overcurrent is detected, then the average power beingdissipated in the heater wire is determined by the time period that thedevice takes to detect the high current (the “on” time) and the periodthat the current is removed (“off”). With the circuit described, themaximum time it will take to detect a high peak current is one AC cycle(i.e. 20 msec at 50 Hz). So long as the “off” time is many times longerthan the “on” time then the heater wire will not dissipate excessivepower and will be safe. In the preferred embodiment the heater wire isturned off for 2 secs, so power is applied for less than 1% of the time.

[0053] The ‘off’ time period should be more than, say, 10 mains cyclesto avoid the power dissipated getting too high. Also it should not betoo long otherwise the operator loses the useful alarm feedback. Forinstance an operator removing the faulty heater wire would expect thehumidifier heater wire alarm condition to cancel promptly. Too long aperiod may confuse the operator.

[0054] Of importance to both spark detection and current detection isthe ability to disconnect the heater wire quickly when one of theseconditions is detected. Conventionally a triac is used to switch offcurrent in an AC circuit such as this, but triacs cannot be turned offinstantly—turn off occurs at the AC zero crossing, which may take up to10 msec at 50 Hz mains. Triacs also have a 1-2 V saturation voltage,which results in power loss. This is to allow the user to connect ordisconnect the heater wires a limited number of times without causing apermanent disconnection or audible alarm. However, repeated sparks wouldcause this to occur.

[0055] Another prior art alternative (shown in FIG. 6) is to use anN-channel MOSFET 31 with a separate substrate connection 32. MOSFETshave a very fast switching time (less than 1 microsecond). They can alsohave a very low “on” resistance (e.g. 0.03 ohm) which results in verylow power dissipation. However a single MOSFET configuration has twodisadvantages. Firstly, it requires a MOSFET with a separate connectionto the substrate, instead of having the substrate 32 connected to thesource 35. Secondly, the substrate 32 must be connected to a biasvoltage 36 which is more negative than the peak negative AC voltage(from the source 39) that will be switched. This is necessary becausethe construction of a MOSFET involves two intrinsic diodes 37,38 betweenthe substrate 32 and the drain 34 and source 35. If these diodes 37,38are not kept reverse biased then they will conduct, and the switchingaction of the MOSFET 31 will be lost. The negative voltage 36 applied tothe substrate 32 keeps these diodes 37,38 reverse biased.

[0056] In the preferred embodiment of this invention (shown in FIG. 7)the heater wire 40 is switched by two back to back N-channel MOSFETs42,43 which have their source connections 44,45 connected together. TheMOSFETs have their substrates 48,49 internally connected to their sourceleads, as is common. As previously described there are intrinsic diodes50,51 connected between the source (substrate) and the drain of eachMOSFET. These diodes are connected back-to-back and do not conduct. Thegates 52,53 of both MOSFETs are connected together, and a voltage 41 isapplied between the gate connections 52,53 and the source connections44,45 to turn the MOSFETs on and off.

[0057] To turn the AC current off the gate-source voltage 41 is set tozero, and the MOSFETs stop conducting. To turn the current on, thegate-source voltage is increased above the threshold voltage of theMOSFET, and they conduct. In the preferred embodiment of the inventionthe “on” resistance is chosen such that at the highest rated current thedrain-source voltage of the MOSFETs never exceeds 0.6V, so that theintrinsic diodes are never allowed to conduct.

[0058] This AC switching configuration overcomes the disadvantages ofthe AC switch in FIG. 6, while still providing a rapid switching time.By using two MOSFETs connected in reverse the intrinsic diodes cannotconduct, without the use of an external negative bias voltage. Also aseparate substrate connection is not required on the MOSFETs.

1. A fault protection system for a respiratory conduit heater elementcomprising: detecting means which include means to detect a rapid changeof current in said heater element, current interruptor means in serieswith said heater element, and timer means adapted to control the actionof said interrupter means and thereby in use determining the duration ofcurrent interruption, said timer means being triggered by said detectingmeans.
 2. A fault protection system for a respiratory conduit heaterelement as claimed in claim 1, wherein said detecting means alsoincludes a peak current detector for detecting current in the heaterelement and said timing means is also triggered by said peak currentdetector when a predetermined threshold current is exceeded.
 3. A faultprotection system for a respiratory conduit heater element as claimed ineither of claims 1 or 2, wherein said detecting means comprises: atransformer including a primary and secondary winding, said primarywinding adapted to be connected in series with said heater element and apower source, a fill wave rectifier including rectifier input means andrectifier output means, said rectifier input means connected across saidsecondary winding of said transformer, a low pass filter includingfilter input means and filter output means, said filter input meansconnected to said rectifier output means, and a comparator includingcomparator input means and comparator output means, said comparatorinput means connected to said filter output means, wherein when theoutput of said low pass filter reaches the set predetermined threshold,said current interrupter means is controlled to interrupt the current insaid heater element.
 4. A fault protection system for a respiratoryconduit heater element as claimed in either of claims 1 or 2, whereinsaid detecting means comprises: a transformer including a centre tappedprimary winding and secondary winding, each end of said primary windingadapted to be connected in series with at least one circuit of saidheater element and said centre tap adapted to be connected to a powersource, a fill wave rectifier including rectifier input means andrectifier output means, said rectifier input means connected across saidsecondary winding of said transformer, a low pass filter includingfilter input means and filter output means, said filter input meansconnected to said rectifier output means, and a comparator includingcomparator input means and comparator output means, said comparatorinput means connected to said filter output means, wherein when theoutput of said low pass filter reaches the first predeterminedthreshold, said current interrupter means is controlled to interrupt thecurrent in said heater element.
 5. A fault protection system for arespiratory conduit heater element as claimed in either of claims 1 or2, wherein said detecting means comprises: an inductor adapted to beconnected in series with said heater element and a power source, a firsthigh pass filter including first filter input means and first filteroutput means, said first filter input means connected to a first end ofsaid inductor, a second high pass filter including second filter inputmeans and second filter output means, said second filter input meansconnected to the other end of said inductor, a full wave rectifierincluding rectifier input means and rectifier output means, saidrectifier input means connected to said first filter output means andsaid second filter output means respectively, a low pass filterincluding filter input means and filter, output means, said filter inputmeans connected to said rectifier output means, and a comparatorincluding comparator input means and comparator output means, saidcomparator input means connected to said filter output means, whereinwhen the output of said low pass filter reaches the first predeterminedthreshold, said current interrupter means is controlled to interrupt thecurrent in said heater element.
 6. A fault protection system for arespiratory conduit heater element, as claimed in either of claims 1 or2 wherein said current interruptor means comprises: two same channelMOSFETS connected in series with their source and gate electrodesrespectively tied together, said circuit adapted to receive a switchingvoltage between the commoned gate and source electrodes.
 7. A faultprotection system for a respiratory conduit heater element, as claimedin either of claims 1 or 2 such that in use when said detecting meansdetermines a fault is present in said heater element said timing meanscontrols said current interrupter means to interrupt the current in saidheater element for a predetermined period after said fault is detected,whereby when said predetermined period expires said timing meanscontrols said current interrupter means to allow current to flow throughsaid heater element.
 8. A semiconductor switching circuit for rapidlycontrolling the AC supply current through a load from a power supplycomprising: two same channel MOSFETs connected in series with theirsource and gate electrodes respectively tied together, said circuitadapted to receive a switching voltage between the commoned gate andsource electrodes, their drain electrodes adapted to be connected to aload and a power supply respectively.
 9. A semiconductor switchingcircuit for rapidly controlling the AC supply current through a loadform a power supply comprising: two same channel MOSFETs connected inseries with their drain and gate electrodes respectively tied together,said circuit adapted to receive a switching voltage between the commonedgate and drain electrodes, their source electrodes adapted to beconnected to a load and a power supply respectively.
 10. A faultprotection system for a respiratory conduit heater element comprising:detecting means which includes a peak current detector for detectingcurrent in the heater element current interruptor means in series withsaid heater element, and timer means adapted to control the action ofsaid interruptor means and thereby in use determining the duration ofcurrent interruption, said timer means being triggered by said detectingmeans, when a predetermined threshold current is exceeded.
 11. In arespiratory humidification system wherein a conduit connects a patientto a humidifier, said conduit being heated by a respiratory conduitheater element controlled by said humidifier, the improvement comprisingthat said humidifier includes a fault protection system for said heaterelement comprising: detecting means which include means to detect arapid change of current in said heater element, current interruptormeans in series with said heater element, and timer means adapted tocontrol the action of said interruptor means and thereby in usedetermining the duration of current interruption, said timer means beingtriggered by said detecting means.
 12. In a respiratory humidificationsystem as claimed in claim 11, the improvements further comprising thatsaid detecting means also includes a peak current detector for detectingcurrent in the heater element and said timing means is also triggered bysaid peak current detector when a predetermined threshold current isexceeded.
 13. In a respiratory humidification system as claimed ineither of claims 11 or 12, the improvements further comprising that saiddetecting means comprises: a transformer including a primary andsecondary winding, said primary winding adapted to be connected inseries with said heater element and a power source, a full waverectifier including rectifier input means and rectifier output means,said rectifier input means connected across said secondary winding ofsaid transformer, a low pass filter including filter input means andfilter output means, said filter input means connected to said rectifieroutput means, and a comparator including comparator input means andcomparator output means, said comparator input means connected to saidfilter output means, wherein when the output of said low pass filterreaches the first predetermined threshold, said current interruptermeans is controlled to interrupt the current in said heater element. 14.In a respiratory humidification system as claimed in either of claims 11or 12, the improvements further comprising that said detecting meanscomprises: a transformer including a centre tapped primary winding andsecondary winding, each end of said primary winding adapted to beconnected in series with at least one circuit of said heater element andsaid centre tap adapted to be connected to a power source, a full waverectifier including rectifier input means and rectifier output means,said rectifier input means connected across said secondary winding ofsaid transformer, a low pass filter including filter input means andfilter output means, said filter input means connected to said rectifieroutput means, and a comparator including comparator input means andcomparator output means, said comparator input means connected to saidfilter output means, wherein when the output of said low pass filterreaches the first predetermined threshold, said current interruptermeans is controlled to interrupt the current in said heater element. 15.In a respiratory humidification system as claimed in either of claims 11or 12, the improvements further comprising hat said detecting meanscomprises: an inductor adapted to be connected in series with saidheater element and a power source, a first high pass filter includingfirst filter input means and first filter output means, said firstfilter input means connected to a first end of said inductor, a secondhigh pass filter including second filter input means and second filteroutput means, said second filter input means connected to the other endof said inductor, a full wave rectifier including rectifier input meansand rectifier output means, said rectifier input means connected to saidfirst filter output means and said second filter output meansrespectively, a low pass filter including filter input means and filteroutput means, said filter input means connected to said rectifier outputmeans, and a comparator including comparator input means and comparatoroutput means, said comparator input means connected to said filteroutput means, wherein when the output of said low pass filter reachesthe first predetermined threshold, said current interrupter means iscontrolled to interrupt the current in said heater element.
 16. In arespiratory humidification system as claimed in either of claims 11 or12 the improvements further comprising that said current interruptormeans comprises: two same channel MOSFETS connected in series with theirsource and gate electrodes respectively tied together, said circuitadapted to receive a switching voltage between the commoned gate andsource electrodes.
 17. In a respiratory humidification system as claimedin either of claims 11 or 12 the improvements further comprising that inuse when said detecting means determines a fault is present in saidheater element said timing means controls said current interruptor meansto interrupt the current in said heater element for a predeterminedperiod after said fault is detected, whereby when said predeterminedperiod expires said timing means controls said current interrupter meansto allow current to flow through said heater element.
 18. In arespiratory humidification system wherein a conduit connects a patientto a humidifier, said conduit being heated by a respiratory conduitheater element controlled by said humidifier, the improvement comprisingthat said humidifier includes a fault protection system for said heaterelement comprising: detecting means which includes a peak currentdetector for detecting current in the heater element current interruptormeans in series with said heater element, and timer means adapted tocontrol the action of said interruptor means and thereby in usedetermining the duration of current interruption, said timer means beingtriggered by said detecting means, when a predetermined thresholdcurrent is exceeded.